This invention relates to a paperboard tray of the type having a polyester coating or layer on its interior, food contacting surface. Such trays are known and are often formed by extrusion coating a layer of polyethylene terephthalate (PET) resin on a paperboard substrate, no adhesive being required due to the affinity of the PET for the paperboard, the resin being applied as an amorphous (noncrystalline) coating. Thereafter, the flat substrate and its coating are deformed or drawn in a reciprocating die apparatus to the shape of a tray useful as the lower portion of an openable container for pre-packaged foodstuffs adapted to be heated in either a microwave or a conventional oven.
Prior art constructions of dual-oven trays of this type relate to extrusion coating with PET as the method of providing the tray stock with a food-contacting coating of heat-resistant plastic. The following U.S. patents discuss this approach: U.S. Pat. Nos. 3,924,013; 4,147,836; 4,391,833 and 4,595,611. This approach has a number of limitations, including: (1) Adhesion of the extruded PET coating can be variable under production conditions, depending on the control of the extrusion coating conditions, the rheology of the molten PET resin, the surface characteristics of the paperboard and other factors. (2) The extruded PET coating is in an amorphous state, and as such can undergo crystallization when heated to conventional oven cooking temperatures and then cooled to room temperature. This gives rise to shrinkage of the coating which causes the trays to warp. Further, the crystallized coating tends to be brittle and may crack during normal handling of the tray. (3) The extruded PET coating is typically not a high-gloss coating. Printing of the paperboard before extrusion coating, when desired to enhance the appearance of the product, must be done with special inks and may require the use of an additional coating over the printed surface to improve the adhesion of the extruded PET coating such as shown in U.S. Pat. No. 4,595,611.
Because of the shortcomings of tray stock made with an extruded PET coating for the food contacting surface, alternate coatings have been considered by workers in this art, but none of these coatings has fully met the requirements for low-cost, dual-oven food trays produced by the pressure-forming, reciprocating die process.
Prior to the present invention, the inventors attempted to produce tray stock by laminating paperboard with various heat-resistant plastic films, including biaxially-oriented polyester film. This film was considered because it is commercially available, also, it is moderately priced, and it has the heat-resistance and other performance characteristics needed in the food-contacting component of dual-oven tray stock. However, our past efforts to use biaxially-oriented polyester film were unsuccessful because the laminated stock exhibited delamination, either immediately after the trays were formed, or later when the trays were subjected to oven cooking temperatures.